1. Field of the Invention
The present invention generally relates to electrochemical systems, and more particularly, to an apparatus and method for managing fluids in a fuel cell stack.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically promotes the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area.
One type of electrochemical fuel cell is a proton exchange membrane (PEM) fuel cell 10 shown in FIG. 2. PEM fuel cells 10 generally employ a membrane electrode assembly (MEA) 5 comprising a solid polymer electrolyte or ion-exchange membrane 2 disposed between two electrodes 1, 3, as shown in FIG. 1. Each electrode 1, 3 typically comprises a porous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane 2 and serves as a fluid diffusion layer. The membrane 2 is ion conductive, typically proton conductive, and acts both as a barrier for isolating the reactant streams from each other and as an electrical insulator between the two electrodes 1, 3. A typical commercial PEM 2 is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®. The electrocatalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support).
As shown in FIG. 2, in a fuel cell 10, the MEA 5 is typically interposed between two separator plates 11, 12 that are substantially impermeable to the reactant fluid streams. Such plates 11, 12 are referred to hereinafter as flow field plates 11, 12. The flow field plates 11, 12 provide support for the MEA 5. Fuel cells 10 are typically advantageously stacked to form a fuel cell stack 50 having end plates 17, 18, which retain the stack 50 in the assembled state as illustrated in FIG. 3.
FIG. 4 illustrates a conventional electrochemical fuel cell system 60, as more specifically described in U.S. Pat. Nos. 6,066,409 and 6,232,008. As shown, the fuel cell system 60 includes a pair of end plate assemblies 62, 64, and a plurality of stacked fuel cells 66, each comprising an MEA 68, and a pair of flow field plates 70a, 70b (generally referred to as flow field plates 70). Between each adjacent pair of MEAs 68 in the system 60, there are two flow field plates 70a, 70b which have adjoining surfaces. A pair of abutting flow field plates 70a, 70b can instead be fabricated from a unitary plate forming a bipolar plate. A tension member 72 extends between the end plate assemblies 62, 64 to retain and secure the system 60 in its assembled state. A spring 74 with clamping members 75 can grip an end of the tension member 72 to apply a compressive force to the fuel cells 66 of the system 60.
Fluid reactant streams are supplied to and exhausted from internal manifolds and passages in the system 60 via inlet and outlet ports 76 in the end plate assemblies 62, 64. Aligned internal reactant manifold openings 78, 80 in the MEAs 68 and flow field plates 70, respectively, form internal reactant manifolds extending through the system 60. As one of ordinary skill in the art will appreciate, in other representative electrochemical fuel cell stacks, reactant manifold openings may instead be positioned to form edge or external reactant manifolds.
A perimeter seal 82 can be provided around an outer edge of both sides of the MEA 68. Furthermore manifold seals 84 can circumscribe the internal reactant manifold openings 78 on both sides of the MEA 68. When the system 60 is secured in its assembled, compressed state, the seals 82, 84 cooperate with the adjacent pair of flow field plates 70 to fluidly isolate fuel and oxidant reactant streams in internal reactant manifolds and passages, thereby isolating one reactant stream from the other and preventing the streams from leaking from the system 60.
As illustrated in FIG. 4, each MEA 68 is positioned between the active surfaces of the flow field plates 70. Each flow field plate 70 has flow field channels 86 (partially shown) on the active surface thereof, which contacts the MEA 68 for distributing fuel or oxidant fluid streams to the active area of the contacted electrode of the MEA 68. The reactant flow field channels 86 on the active surface of the flow field plates 70 fluidly communicate with the internal reactant manifold openings 80 via reactant supply/exhaust passageways comprising back-feed channels 90 located on the non-active surface of the plate 70 and back-feed ports 92, extending through (i.e., penetrating the thickness) the plate 70, and transition regions 94 located on the active surface of the plate 70. As shown, with respect to one port 92, one end of the port 92 can open to the adjacent back-feed channels 90, which can in turn be open to the internal reactant manifold opening 80, and the other end of the port 92 can be open to the transition region 94, which can in turn be open to the reactant flow field channels 86.
Instead of two plates 70a, 70b, one plate 70 unitarily formed or alternatively fabricated from two half plates 70a, 70b can be positioned between the cells 66, forming bipolar plates as discussed above.
The flow field plates 70 also have a plurality of typically parallel flow field channels 96 formed in the non-active surface thereof. The channels 96 on adjoining pairs of plates 70 cooperate to form coolant flow fields 98 extending laterally between the opposing non-active surfaces of the adjacent fuel cells 66 of the system 60 (i.e., generally perpendicular to the stacking direction). A coolant stream, such as air or other cooling media may flow through these flow fields 98 to remove heat generated by exothermic electrochemical reactions, which are induced inside the fuel cell system 60.
In the conventional fuel cell system 60, water typically accumulates in the flow field channels 86, back-feed channels 90 and/or back-feed ports 92. As gas, such as reactants and/or oxidants, is injected into the flow field channels 86, the gas pressure and movement may flush some of the accumulated water through the above-described outlets.
If a relatively large amount of water collects in a localized region of the flow field channels 86, back-feed channels 90 and/or back-feed port 92, the water may block the channels 86, 90 or port 92. If the accumulated water blocks the channels 86, 90 or port 92, gas flow can be adversely affected, and in extreme cases, cease. Consequently, as the reactants and/or oxidants in the gas residing in the blocked channels 86, 90 or port 92 are depleted, electrical output and fuel efficiency of the fuel cell decreases.
Such water accumulation can also lead to ice formation before and during freeze startups. Although purging the water from the system is one option for preventing water accumulation, regions of low purge velocity tend to retain water during a purge. Furthermore, due to the large ratio of capillary forces from the back-feed port 92 to the reactant manifold openings 78, water tends to wick back into the exit of the back-feed port 92 after the purge. Therefore, after the purge, regions of low purge velocity in the reactant manifold openings 78 typically store relatively large amounts of water, which may wick or otherwise move back into the back-feed channels 90 and/or back-feed port 92. This water can freeze, resulting in ice blockage. These blockages typically prevent efficient reactant access and flow to the flow field channels 86 and may cause uneven flow sharing and/or fuel starvation in the fuel cell system 60.
In addition to purging the water from the system 60, other methods of mitigating ice blockages include operating the fuel cell system 60 extremely dry; however, even then, some water accumulation and/or ice blockage occurs because it is nearly impossible to completely prevent water from exiting the fuel cells 66. Furthermore, operating fuel cell systems in extremely dry conditions typically impedes performance and reduces the fatigue life of the system 60.
Those of ordinary skill in the art will appreciate that other configurations for the reactant supply manifolds and back-feed channels and ports exist, nearly all of which suffer from the above obstacles. For example, FIG. 5 illustrates a front view of a non-active side of a flow field plate 100 of another conventional system. Reactant back-feed channels 102 and ports 104 are prone to water formation and ice blockage as described above. FIG. 5 more clearly conveys the adverse effect of ice blockage in these channels 102 and ports 104 on the operation of the fuel cell system because if these channels 102 and ports 104 are blocked or even partially obstructed, reactants such as fuel and oxidants cannot efficiently reach the active side of the flow field plate 100 to support reactions necessary for the system to operate efficiently.
Accordingly, there is a need for an apparatus and method for managing fluid flow in a fuel cell stack that substantially prevents water retention and ice-blockage formation in the fuel cell stack, that is inexpensive, space conserving and easy to implement.